Now to that alarming problem overtaking some of america's lakes. We even saw it in washington, d.C. This week at the lincoln memorial where the famous reflecting pool was instead filled with clumps of green. It is happening all across america making some people very sick. Here's abc's senior national correspondent jim avila tonight. Reporter: It's an assault on nearly all the senses -- a living, breathing, growing toxic organism known as blue-green algae -- at its worst this month. That's very much alive. Gosh. It smells bad. Whoo. Reporter: Caking america's lakes so thick, the swirling slime can actually be seen in photos from space. People that used to be here after a couple years said, we just can't handle it anymore, they left. Reporter: Tom koren runs the marina at lake petenwell, wisconsin, or tries to. Nobody wants to swim in this water, and nobody wants to boat in it. Reporter: It not only looks bad and smells bad, it's toxic, and those who are unlucky enough to fall in without a suit like this get sick pretty fast. Dan jenkins didn't even go in the lake next to his house, but his dog, casey, did. He's covered in this carpet of green slime. Reporter: It made breathing difficult, and within weeks -- casey had started walking sideways. Reporter:72 hours later, he died, and his owner, dan, was partially paralyzed. Oh, it hit me hard. Wobbling around like that, it would make me mad. I mean, it was a fight. Reporter: He still hasn't recovered, and a joint investigation by abc news and the food and environment reporting network found more than 100 reported illnesses due to blue-green algae exposure. Essentially if we don't solve this pblem, somebody's going to die. Reporter: Ohio state's dr. Jeffery reutter is the foremost authority on blue-green algae. He says it's a nationwide problem, largely caused by farm fertilizer runoff. The cure, he says, is convincing farmers to carefully fertilize so the nutrients stay on the fields and not in the water. Otherwise, these blooms are going to continue to grow. The human health problems that we see are going to increase. Reporter: Assaulting our senses, our economy and our health. Jim avila, abc news, lake petenwell, wisconsin. Our thanks to jim tonight.

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